In the manufacture of microcellular polyurethane elastomers from polyesters for shoe soles, it has been virtually impossible to perfect a suitable technique for the molding of products which have an acceptable peripheral outer skin (integral skin). The manufacture of skins of this type is important for lacquering of soles to obain asthetically acceptable and durable products. Moreover, the skin should be impervious to water or oil, and should be free of surface blisters, voids, and other blemishes.
Most of the microcellular urethane elastomers are produced from a polyester and an isocyanate employing water or mixtures of water and organic solvents as blowing agents (reaction of water with isocyanate yields carbon dioxide), to obtain the desired expansion of the polymer to yield a product having a density of from about 20 to 50 lbs./cu. ft. (Specific gravity about 0.2 to 0.8 gm/cm.sup.3). When water is used as the reactant for blowing, the resulting product is microporous up to and including the skin itself. This type of product is much more difficult to paint or coat because it contains a fairly porous surface, necessitating the use of sealing primers prior to painting.
The manufacture of integral-skin products has several advantages including direct paintability without primer, better flex life, because of higher resistance to growth from a cut inflicted in the dense skin (known as "cut growth"), much higher tear strength of the skin, and generally superior abrasion resistance.
It has been observed that during the reaction of polyethers, glycol chain extenders and aromatic polyisocyanates (for example prepolymers of p,p'-diphenylmethane diisocyanate), in the presence of fluorocarbon blowing agents and appropriate catalysts, it is feasible to manufacture integral-skin microcellular polyurethanes having suitable surfaces. Although these products are employed in the manufacture of shoe soles, these products are somewhat deficient in wear and abrasion resistance and also with respect to oil and solvent resistance. Consequently, these products are less useful, in general, and especially for athletic or work shoes than the polyester based products.
Prior to the present invention, the manufacture of integral-skin microcellular polyurethane elastomers from polyesters has been difficult (apart from the obtaining even of a satisfactory skin). Polymerization of polyesters, glycol chain extenders, and prepolymers from p,p'-diphenylmethane diisocyanate in the presence of fluorocarbon blowing agents and appropriate catalysts has caused many difficulties, since the resulting product has a normally closed cell structure and will shrink upon cooling. Cell control agents, such as polydimethyl siloxanes or polyphenylmethyl siloxanes, are helpful in this respect, as are mineral oils and crude oils, but these have had the disadvantage of yielding surface blemishes such as voids or craters near or just below the product surface.